FINE GRINDING.

237

The speed of revolution of tube mills varies with the diameter. Krupp

uses the formula / - for the correct number of revolutions per minute,

VD . 200

where D = internal diameter in metres. The Davidson formula is / ,

where D is the diameter in inches. These two formula are almost identical and would give about 26 revolutions per minute for a mill of 5 feet in diameter.

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White's formula is '•-—, where D is in metres. The speed in Rand tube

VD

mills varies from 28 to 34 revolutions per minute with a diameter inside the shell of 5 feet 6 inches.1 The peripheral speed inside the lining at 32 revolutions with new linings 6 inches thick would be 452 feet per minute, and with worn linings only 1 inch thick it would be 536 feet per minute. As the lining-wears, the number of revolutions per minute is reduced to keep the peripheral speed approximately unchanged. These are higher speeds than wore formerly used, and somewhat higher than that generally used elsewhere, but the speed is dependent to some extent on the other conditions. There is a

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Fig. 105.—Pryco Pulp and Pohhlo Km lor, with TIummM'B Uuppor.

certain peripheral rate of speed which is most advantageous with a par ticular ore. The poiw.r required varies with the rate of food (load) and speed. A standard Rand tube mill of 22 feet long by 5,1 feet diameter requires from 90 to 120 II.P., increasing as the liners wear, owing to increased size and capacity of tho. mill. Motors of 125 11. P. are used, but these are to be increased to 175 I I.I', in new mills. According to Davidson the formula J? (in II.P.) 0-15 N, where N onpacity of mill in cubic, feet, gives the power required, but this is too low for Rand practice,

The capacity (output) of tube mills depends on the hardness and sixo of the material supplied to them, and on the fineness of the product. The amount of grinding done and the fineness of the product depend on the rale of feed and the length of the tube, as well as on the hardness of tho ore. According to Gruessner,2 at the Jlannaifs Star Mill, Kalgoorlio, the ore from

1 Dowlirif?, Rand Mct<iUurtfi('<({ Practice vol. i., i>. 117. Hco al«o Hall, '/'run*, /nut. Mntt and Met., 1912, 21, 3.

2 Gruessner, Tram. In«t. AT//.//, and Met., 11)04, 14, 87.